1. Technical Field
The present invention relates to a technique of forming dots on a printing medium to print an image.
2. Related Art
Printing devices that form dots on a printing medium to print an image have been used widely as the output device of images generated by computers and images taken with digital cameras. The printing device conventionally performs halftone process by the known dither method or the error diffusion method to specify the dot formation state on the printing medium.
However, the conventional dither method and error diffusion method have intrinsic limitations. The dither method directly controls the dot formation state by appropriate arrangement of threshold values stored in a dither matrix, but has a limitation in arrangement of the threshold values in the dither matrix due to the requirement for consideration about the whole range of expected input tone values. The optimized arrangement of threshold values in a certain range of input tone values causes a significant deviation from the optimum state in another range of input tone values. Some compromise is thus required for arrangement of the threshold values in the individual ranges of input tone values. The error diffusion method, on the other hand, disperses dots by diffusion of the arising errors and thus attains the good dispersibility of dots without such compromise. The error diffusion method, however, has disadvantages of difficulty in direct control of the dot formation state and of the severe processing load.